Large field reduction lens system



Nov. 17, 1970 Filed Jqly 15. 1968 FIG.5

% NOOULATION TRANSFER FUNCTION 4o FIG. 6

% MODULATION TRANSFER ruucnou 40 FIG; 7

/ MODULATION TRANSFER FUNCTION 40 R. E. TIBBETTS ETAL I LARGE FIELD REDUCTION LENS SYSTEM 6 Sheets-Sheet I5 3,; LINES SPATIAL FREQUENCY 687 LINES/ MM A 45AZII4UTH SACITTAL .TANCENTIAL (REAL PART) I!INCENT|AL (IMACINARY PART) 687 LINES/NM SPATIAL FREQUENCY 687 LINES/MM SPATIAL FREQUENCY Nov. 17, 1970 R. E. TIBBETTS ETAL 3,540,800

LARGE FIELD REDUCTION LENS SYSTEM Filed July 15, 1963 6 Sheets-Sheet 4 100 F I 8 DIFFRACTION 80 s1=-.1o3m4 MODULATION 60 TRANSFER FUNCTION 20 5p, LINES 0 l l N l l l l SPATIAL FREQUENCY 416 UNEs/MM FIG. 9

A AZ|NUTH SAGITTAL so J R EE M /o MODULATION (REAL PART) TRANSFER FUNCTION 40 TANGENHAL (IMAGTNARY PART) 0 ll L; I; l

SPATIAL FREQUENCY 41s LINES/MM FIG. 1C

vouonuunou TRANSFER ruucnon 40 0 l l 1 l l l l I SPATIAL FREQUENCY LINES/MM Nov. 17, 1970 R. a. TIBBETTS ETAL 3,

LARGE FIELD REDUCTION LENS SYSTEM Filed July 15, 1968 6 Sheets-Sheet 5 +15 FULL FIELD PHASE ANGLE 0 I 3 I I I 1' 687 LINES/M SPATIAL FREQUENCY Ykflll) ASTIGMAHSI DISTORTION FIG. 12 FIG. 13

Nov. 17, 1970 R. E. TIBBETTS ETAL LARGE FIELD REDUCTION LENS SYSTEM Filed July 15, 1968 6 Sheets-Sheet 6 FIG. 14

0.7 FIELD 416 LINES/MM FULL FIELD SPATIAL FREQUENCY -1o Yk um) YkUIM) 1 sl'um 1 -L -.12 -.1o -.oa -.oo5% o +.oo5%

Asncnmsu msronnou F|G.15 FIG. 16

United States Patent Oifice 3,540 800 LARGE FIELD REDUCTION LENS SYSTEM Raymond E. Tibbetts, Mahopac, and Janusz S. Wilczyuski,

Ossiuing, N.Y., assignors to International Business Ma-' chines Corporation, Armonk, N.Y., a corporation of New York Filed July 15, 1968, Ser. No. 744,830 Int. Cl. G02b 9/00 U.S. Cl. 350-214 4 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to optical lens systems and more particularly, to a lens group for use as a reduction lens. The lens system consists of nine lens elements. The first and second lens elements are meniscus singlet lenses. The third and fourth lens elements arecemented together to form a meniscus doublet lens. The fifth lens element is a negative singlet lens. The sixth and seventh lens elements are cemented together to form a meniscus doublet lens. Lens element eight is a meniscus single lens and lens element nine is a biconvex lens. The

BACKGROUND OF THE INVENTION Field of the invention I The field of art to which the present invention pertains is that of optical lens systems and elements.

SUMMARY OF THE INVENTION The present invention relates to a lens group for use as a reduction lens. Reduction lenses of high quality are useful for many applications, particularly in the fabrication of semiconductor integrated circuits by optical methods wherein circuit element images are reduced and projected onto the plane surface of semiconductor wafers. Reduction lenses of this type are required to have uniform resolution over the image field and exceedingly low astigmatism together with minimum field curvature and negligible distortion. It is also very desirable that the reduction lens cover a relatively large image field. Heretofore, the maximum wafer diameter that could be easily covered and yield high quality circuits was in the order of 32 millimeters. In the present invention, two embodiments are provided. The first embodiment is a lens system for covering a wafer of 50 millimeter diameter and the second embodiment covers a wafer of 110 millimeter diameter.

The lens group of the first embodiment will yield 3 mi cron line widths at 50% contrast (4047 A.) over a 50 mm. diameter wafer. The total bit content would be approximately 2.l8 1() bits of 3 micron square elemental bit size. The lens group of the second embodiment will yield micron line widths at 50% contract (5461 A.) over 3,540,800 Patented Nov. 17, 1970 a mm. diameter wafer. The total bit content would be approximately 3.8)(10 bits of 5 micron square elemental bit size. The line widths at 50% contrast stated in the above two embodiments are minimal values in the tangential, sagittal, and skew meridians.

To ensure a steep edge gradient, the phase angle must remain small at all frequencies to the limiting value since all frequencies are superimposed in the formation of the edge of a line, block or the like. If a microcircuit having 3a lines and 3 spaces in some intermingling geometric arrangement is considered, the clear portions should be as clean as possible to increase the percentage of usable circuits. Therefore, the third harmonic of the desired fundamental frequency should exist and have a very small phase angle. Since both embodiments have at their respective third harmonics phase angles of less than 7 de grees, the shift of the third harmonic from the fundamental harmonic is only A of the formers period, effecting an extremely steep edge gradient at the fundamental frequency since the third harmonic will also have the tendency to increase the gradient of the fundamental frequency alone.

The high quality of the lens embodiments is indicated by curves showing the modulation transfer function, the phase angles, the astigmatism, and the *distortion'of the lens systems.

It is an object therefore of the present invention to provide a reduction lens which has a relatively large image field and has high uniform resolution over the large field, has extremely low astigmatism and minimum field curvature with negligible distortion.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 shows an optical diagram of a reduction lens system constructed according to the present invention;

FIG. 2 shows a chart of the constructional data for the lens system of FIG. 1;

FIG. 3 shows an optical diagram of another embodiment of a reduction lens system constructed according to the present invention;

FIG. 4 shows a chart of the constructional data for the lens system of FIG. 3;

FIGS. 5, 6 and 7 show curves of the modulation transfer function of the lens system of FIG. 1 for the lens axis, for 0.7 field, and for full field respectively;

FIGS. 8, 9 and 10 show curves of the modulation transfer function of the lens system of FIG. 3 for the lens axis, for 0.7 field, and for full field respectively;

FIGS. 11, 12 and 13 show curves of the phase angles, residual field curvature and distortion respectively for the lens system of FIG. 1;

FIGS. 14, 15 and 16 show curves of the phase angles, residual field curvature and distortion respectively for the lens system of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, an optical diagram of an embodiment of a reduction lens according to the principles of the present invention is shown. The lens group consists of nine lens elements designated as lens elements I, II, III, IV, V, VI, VII, VIII and IX. Lenses I and II are meniscus singlet lenses. Lenses III and IV form a meniscus cemented doublet lens. Lens V is a negative meniscus singlet lens and lenses VI and VII form a meniscus doublet lens. Lens VIII is a meniscus singlet lens and lens IX is a biconvex lens. The lenses are optically aligned on an axis to have an effective focal length of 184.0 mm., a back focal length of 64.9 mm. and a front focal length of 81.7 mm. A fixed stop or diaphragm 12 of 32.6 mm. diameter at F/3 is located 3.8 mm. to the left of lens element V. 1

Referring to FIG. 2, a table of the constructional data related to the lens group shown in FIG. 1 is set forth. This constructional data is also set forth in Table 1 as follows:

TABLE 1 [173; Reduction=5.0 l

Lens Radius Thickness (t) or Airspace (S) VII 54. 80

VIII 1. 69089 In Table 1, F represents the equivalent focal length of the lens group (184.0 millimeters) at 4047 A. R through R represent the radii of the successive lens surfaces as indicated in FIG. 1. t through t designate the thicknesses of the successive lenses measured along axis 10 as indicated in FIG. 1 and S through 8,; represent the spacing between the lenses I and II, II and III, IV and V, V and VI, VII and VIII, and VIII and IX measured along axis 10 as indicated in FIG. 1. Also set forth in FIG. 2 are the designations N and V which represent respectively the refractive index and the Abbe number of each lens element. The lens system shown in FIG. 1 is capable of covering a linear image field of 50 millimeters diameter.

Referring to FIGS. 5, 6 and 7, the curve of the modulation transfer function of the lens system of FIG. 1 over a linear image field of plus or minus 25.0 millimeter diameter at a reduction of 5.0x is shown. For the axis of the lens, 0.7 field, and for full field, respectively, all calculated at one focal setting of -0.*064 millimeters. FIGS. 5, 6 and 7 indicate that the departure of the curves from the diffraction limit is extremely small for the relatively large diameter image field. The similarity of the curves in FIGS. 5, 6 and 7 should he noted since this provides high uniform performance across the large field.

In FIGS. 6 and 7 the sagittal fan, the tangential fan and the imaginary parts of the tangential fan and 45 azimuth are shown for 0.7 of the field and for full field. The imaginary parts of the tangential fan are exceedingly small indicating the wavefronts which immerge from the lens system at all points in the field have almost perfect rotational symmetry about their respective principal rays.

gential fan. The 45 azimuth response is superimposed on the tangential curve effecting emerging wavefronts still closer to the desired spherical shape. FIG. 11 shows the phase angle for 0.7 of the field and for full field. Small values of all phase angles hence minimal coma with its resulting smear of off-axis images are very important in reduction lenses used in the production of microcircuits, since clean circuits are necessary.

The excellent correction of the wavefront cross-section at 45 angle in the exit pupil requires particular explanation. It results from independent correction of tangential oblique spherical aberration and astigmatism. This state is further enhanced by high degree of elimination of the particular coma term that is represented only in the tangential section and whose partial derivative with respect to the sagittal exit pupil coordinate is zero. This term which grows as third power of the fractional image height and third power of the fractional tangential pupil height in this particular embodiment is reduced to a few hun dredths of a wavelength. In addition, this coma term is balanced with others growing also as cube of the fractional image height. Needless to say that both lenses obey almost exactly the Abbe sine conditions leaving therefore no appreciable residuals of coma in any part of the field.

The residual field curvature for the lens system of FIG. 1 is illustrated in FIG. 12 and is well within the allowable focal range of 21 microns, the astigmatism is exceedingly small, less than 2 microns.

FIG. 12 illustrates the distortion and indicates that it is of such small magnitude as to be nearly unmeasurable and of such a balanced state as to yield the best orthoscopic results, the maximum radial spatial error being 0.5 microns.

Referring to FIG. 3, another embodiment of a lens system according to the principles of the present invention is shown. In the embodiment of FIG. 3, the same number and form of lens elements are provided and arranged as described for the embodiment of FIG. 1. Therefore, the same reference numerals will be employed in FIG. 3 corresponding to the similar lens elements in FIG. 1. Thus, the lens system of FIG. 3 consists of nine lens elements, lenses I and II are meniscus singlet lenses, lenses III and IV form a meniscus cemented doublet lens, lens V is a negative singlet lens and lenses VI and VII form a mensiuus doublet lens, lens VIII is a meniscus singlet lens and lens IX is a biconvex lens. The lenses are optically aligned on an axis 10 to have an effective focal length of 385.0 mm., a back focal length of 176.3 mm., a front focal length of 193.5 mm., a fixed stop or diaphragm 12 of 55.0 mm., diameter at FM is located 4.0 mm. to the right of lens element V. The lens system of FIG. 3 has a reduction of 10x over a linear image field of plus or minus 55 millimeters in diameter. A table of constructional data for the lens system of FIG. 3 is shown in FIG. 4. It is noted that corresponding lens elementsof the lens system of FIG. 3 each have the same refractive index N and Abbe number V as those in FIG. 1. The radius of curvature, the thickness of the lens and the spacing of the lens elements are set forth in FIG. 4 and indicate the extent to which the embodiment of FIG. 3 differs from the embodiment of FIG. 1. The modulation transfer function curves of the lens system of FIG. 3 for the axis for .7 field and for full field are shown respectively in FIGS. 8, 9 and 10. The calculations for FIGS. 8, 9 and 10 were done at a focal length of 385.0 millimeters at 5461 A. at a reduction of 10X over a linear image field of plus or minus 55 millimeters, all calculated at one focal setting of .103 millimeters. The constructional data of FIG. 4 is also set forth in Table 2, as follows:

TABLE 2 [f/4; Reductlon= 10X] Lens Radius Thickness (t) N V or Airspace (S) R1= +1.1595F I t1=0.0416F 1. 69089 54. 80

Rz=+LB344F S1= 0.0052]? R3= +0.4309F II ts=0.057IF 1. 69089 54. 80

' S2=0.0052F Rs= +0.33l9F III t3=0.0922F I. 69089 54. 80

Rs= +2.5044F IV =0.0377F 1. 64752 33. 88 R7= +0. 2075F S3=0.1065F R3 32.94413 V ts=0. 0260F 1. 64752 33. 88

S4= 0.1408F Rw= .2729]? VI ta= 0.0377F 1. 003% 38. 02

Rn 1.3645 VII t1=0.0909F 1. 69089 54. 80

R z= 0.4587F Ss= 0.00521! R s= -0.6899F VIII tg=0.0442F I. 69089 54. 80

Ss=0.0052F R15= +2.2256F IX q=0.0571F 1. 69089 54. 80

R1a= -0.9501F FIG. 8 illustrates that the departure from the diffraction limit is extremely small. FIGS. 9 and 10 illustrate that the imaginary parts of the tangential fans are also exceedingly small indicating a most perfect rotational symmetry of the wavefronts around their respective principal rays. The 45 azimuth response is again superimposed on the tangential curves.

FIG. 14 illustrates the residual phase angles of the lens system of FIG. 3. The same can be said for the coma correction of this lens as was stated above about the coma correction of the embodiment of FIG. 1.

FIG. 15 illustrates the residual field curvature of the lens system of FIG. 3 and indicates that the astigmatism is very small, well within the allowable focal range of 42 microns FIG. 16 illustrates the distortion of the lens system of FIG. 3 is also of extremely small magnitude and of a balanced state, such that the maximum radial spatial error is 1.1 micron.

The constructional data set forth in FIGS. 2 and 4 can vary between limits to some extent. The tolerances for the variations of the radii, the thickness and the airspaces for the lens system of FIG. 1 are as follows.

The tolerances for the variations of the radii, the thicknesses and the airspaces of the lens system of FIG. 3 are as follows.

The tolerances of the radii are approximately 15% since each radii could vary within these limits and yet produce a lens system as well corrected as those specified in FIGS. 2 and 4.

The thicknesses vary differently. The range of thicknesses vary from lens to lens and range from most critical to less critical in the following order.

The tolerances of the thicknesses are of concern in the fabrication of the lens elements. For example, the tolerances of elements I and V are not as critical as the tolerance of element III.

The airspace tolerances are determined in the same manner but can be less critical because an equivalent lens system can be produced with lens elements -I, 11, VIII and IX further away from the doublet elements. Airspaces 8;; and S are more critical and cannot be changed too much because the oblique spherical aberration of the tangential fan is corrected mainly in the space between the two doublet elements.

In addition to the aforesaid application as reduction lenses for microcircuit fabrication, the lens system of FIG. 1 can also be employed in high quality scanner printer devices where the line widths as narrow as 5 microns are required.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

. What is claimed is:

1. A reduction lens group comprising first, second, third, fourth, fifth, sixth, seventh, eighth, and ninth optically aligned lens elements wherein said first lens element has radii of curvature R and R, and thickness t said second lens element has radii of curvature R and R and thickness t said third lens element has radii R and R and thickness t said fourth lens element has radii R and R and thickness t said fifth lens element has radii R and R and thickness t said sixth lens element has radii R and R and thickness t -said seventh lens element has radii R and R and thickness t said said seventh and eighth lens elements are separated by an axial distance S and said eighth and ninth lens elements are separated by an axial distance S and wherein the radii, thicknesses and distances of lens elements are within the limits as a function of the focal length E of 184.0 millimeters as follows:

Lens Radius Thickness (t) N D V or Airspace (S) R +1.2090F I t =0.0543F 1. 69089 54. 80

Ri= +2.9152F Si=0.0054F R +0.4459F II t 0.0679F 1. 69089 54. 80

Ri= +0.953OF Sz= 0.0054]? R +0.3691F III t =0.1033F 1. 69089 54. 80

R6= +1.3253F IV t =0.0435F 1. 64752 33. 88

Ss= 0.1408F Ra= +6.3772F V t =0.0326F 1. 64752 33. 88

Ra= +2.2438F Si=0.l141F Rm: 0.2775F VI t==0.0435F 1. 60328 33. 02

Rii= -0.6014F VII t 0.0870F 1. 69089 54. 80

Ri2= -A394F S 0.0054F Ria= 0.7768F VIII t =0.0543F 1. 69089 54. 8

Si=0.0054F R +1.3292F IX t =0.0543F 1. 69089 54. 80

wherein F represents the efiective focal length of the lens system of 184.0 millimeters at 4047 angstroms,

and wherein R through R represent the radius of curvature of the individual lens elements, t through t represent the axial thickness of the individual lens elements, S through 8,, represent the axial spacing of the individual lens elements, N represents the refractive index of the individual lens elements and V represents the Abbe number of the individual lens elements.

3. A reduction lens group comprising first, second, third, fourth, fifth, sixth, seventh, eighth, and ninth optically aligned lens elements wherein said first lens element has radii of curvature R and R and thickness t said second lens element has radii of curvature R and R and thickness t said third lens element has radii R and R and thickness t said fourth lens element has radii R and R and thickness t said fifth lens element has radii R and R and thickness t said sixth lens element has radii R and R and thickness i said seventh lens element has radii R and R and thickness 1 said eighth lens 2. A lens group according to claim 1 having numerical data substantially as follows:

8 arated by an axial distance S said fifth and sixth lens elements are separated by an axial distance S said seventh and eighth lens elements are separated by an axial distance S and said eighth and ninth lens elements are separated by an axial distance 5,,

4. A lens group according to claim 3 having numerical data substantially as follows:

Thickness) Lens Radius or Airspace (S) N V Ri=+1.1595F I t =0.0416F 1.69089 5480 R2=+L6344F SIFO-W52F R@=+0.4309F II t,=0.0571F 1.69089 54.80

Ri=+0.9905F S2=0.005ZF R5=+0.3319F III ,=0.0922F 1. 69080 54.80

R=+2.5044F IV R 5F t =0.0377F 1. 64752 33.88

R,=-32.044F V i,=0.0260F 1.64752 33.88

R =-l-5.8480F S4=0.1408F R|0=-0.2729F VI z,=0.0377F 1.60328 38.02 R -1.3645

VII t =0.0909F 1.69089 54.80

IMF-0.4587]? S5=0.0052F R1s=0.6899F VIII t =0.0442F 1. 60080 54.80

Ri4=--0.6287F s,=0.00521 R15=+2.2256F IX t,=0.0571F 1.60089 54.80 RIF-0.05011 wherein F represents the effective focal length of the lens group of 385.0 millimeters at 5461 angstroms, and wherein R through R represent the radius of curvature of the individual lens elements, t through t represent the axial thickness of the individual lens elements, S through S represent the axial spacing 0 of the individual lens elements, N represents the refractive index of the individual lens elements and V represents the Abbe number of the individual lens elements.

References Cited UNITED STATES PATENTS 2,48l,639 9/1949 Altman et a1. 350-2l4X 2,746,351 5/1956 Tronnier 350-207X 2,846,923 8/ 1958 Tronnier 350--2l4 element has radii R and R and thickness t and said ninth lens element has radii R and R and thickness and wherein said first and second lens elements are separated by an axial distance S said second and third lens elements are separated by an axial distrance 5;, said fourth and fifth lens elements are sep- DAVID SCHONBERG, Primary Examiner 

